The present invention relates to microelectronic substrate packages having a pre-disposed fill material for mounting the package to a supporting member.
Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in a plastic, ceramic or metal protective covering. The die includes functional devices or features, such as memory cells, processor circuits and interconnecting wiring. The die also typically includes bond pads electrically coupled to the functional devices. The bond pads can be coupled to pins or other types of terminals that extend outside the protective covering for connecting to buses, circuits, and/or other microelectronic assemblies.
One conventional xe2x80x9cflip chipxe2x80x9d package 10 shown in plan view in FIG. 1 includes a microelectronic die 20 having a downwardly facing surface 24 with solder ball pads 22, and an upwardly facing surface 23 opposite the downwardly facing surface 24. Solder balls 21 are attached to the solder ball pads 22 and dipped in flux. The die 20 is then positioned with the downwardly facing surface 24 facing toward a printed circuit board (PCB) 30 to engage the solder balls 21 with corresponding bond pads 31 on the PCB 30. The solder balls 21 are partially melted or xe2x80x9creflowedxe2x80x9d and solidified to form structural and electrical bonds with the bond pads 31 on the PCB 30.
In one aspect of the arrangement shown in FIG. 1, a gap corresponding roughly to the diameter of the solder balls 21 remains between the upper surface of the PCB 30 and the downwardly facing surface 24 of the die 20 after the die 20 has been attached. The gap can be detrimental to the integrity and performance of the die 20 because it can allow oxidizing agents and other contaminants to attack the solder ball bond between the die 20 and the PCB 30. Furthermore, the gap can reduce the rate at which heat is transferred away from the die 20, reducing the life expectancy and/or the performance level of the die 20.
To alleviate the foregoing drawbacks, an underfill material 40 is typically introduced into the gap between the die 20 and the PCB 30. For example, in one conventional approach, a bead of flowable epoxy underfill material 40 is positioned on the PCB 30 along two edges of the die 20. The underfill material 40 is heated until it flows and fills the gap by capillary action, as indicated by arrows xe2x80x9cAxe2x80x9d. The underfill material 40 can accordingly protect the solder ball connections from oxides and other contaminants, and can increase the rate at which heat is transferred away from the die 20. The underfill material 40 can also increase the rigidity of the connection between the die 20 and the PCB 30 to keep the package 10 intact during environmental temperature changes, despite the fact that the die 20, the solder balls 21 and the PCB 30 generally have different coefficients of thermal expansion.
One drawback with the capillary action approach described above for applying the underfill material 40 is that the underfill material 40 can take up to 20 minutes or longer to wick its way to into the gap between the die 20 and the PCB 30. Accordingly, the capillary underfill process can significantly increase the length of time required to produce the packages 10. One approach to addressing this drawback (typically referred to as a xe2x80x9cno-flowxe2x80x9d process) is to first place the underfill material directly on the PCB 30 and then place the die 20 on the underfill material. For example, as shown in FIG. 2A, a quantity of underfill material 40a having an integrated quantity of flux can be disposed on the PCB 30 adjacent to the bond pads 31. As shown in FIG. 2B, the die 20 can be lowered onto the PCB 30 until the solder balls 21 contact the bond pads 31 of the PCB 30. As the solder balls 21 approach the bond pads 31, the die 20 contacts the underfill material 40a and squeezes the underfill material 40a outwardly around the solder balls 21 and between the downwardly facing surface 24 of the die 20 and the upper surface of the PCB 30, as indicated by arrows xe2x80x9cBxe2x80x9d. An encapsulating material 70 is then disposed on the die 20 and the PCB 30.
One problem with the no-flow process described above with reference to FIGS. 2A-2B is that air bubbles can become trapped between the die 20 and the PCB 30. The air bubbles can reduce the effective bond area between the die 20 and the PCB 30 and can make the die 20 more likely to separate from the PCB 30. Furthermore, oxygen in the air bubbles can oxidize the connection between the solder balls 21 and the solder ball pads 22 and/or the bond pads 31 to reduce the integrity of the structural and/or electrical connections between the die 20 and the PCB 30.
Another problem with the process described above with reference to FIGS. 2A-2B is that it can be difficult to accurately meter the amount of underfill material 40a applied to the PCB 30. For example, if too little underfill material 40a is provided on the PCB 30, the solder balls 21 may not be adequately covered. Even if the underfill material 40a extends beyond the solder balls 21 to the edge of the die 20 (as indicated in dashed lines in FIG. 2B by position P1), it can exert a tensile force on the die 20 that tends to separate the die 20 from the PCB 30. Conversely, if too much underfill material 40a is provided on the PCB 30, the underfill material can extend over the upperwardly facing surface 23 of the die 20 (as indicated in dashed lines in FIG. 2B by position P2), and can form protrusions 49. The protrusions 49 can be subjected to high stress levels when the die 20 is encapsulated with the encapsulating material 70, and can cause the underfill material 40a to separate from the die 20. Still further, the underfill material 40a can become trapped between the solder balls 21 and the bond pads 31 and can interfere with the electrical connections between the die 20 and the PCB 30.
The present invention is directed toward microelectronic device packages and methods for forming such packages by bonding microelectronic substrates to support members, such as PCBs. A method in accordance with one aspect of the invention includes disposing a fill material in a fill region defined by a surface of the microelectronic substrate before engaging the fill material with the support member. The fill region can also be defined in part by a bond member (such as a solder ball) or other protrusion projecting away from the surface of the microelectronic substrate. The method can further include engaging the fill material with the support member after disposing the fill material in the fill region, and connecting the bond member and the fill material to the support member. The microelectronic substrate and the fill material can then be at least partially enclosed with an encapsulating material.
In one aspect of the invention, the microelectronic substrate is dipped into a vessel of fill material and is then removed from the vessel with a portion of the fill material attached to the surface of the microelectronic substrate. Accordingly, the fill material can have a thixotropic index with a value of from about four to about six. In another aspect of the invention, the surface of the microelectronic substrate can be a first surface and the microelectronic substrate can include a plurality of second surfaces extending away from the first surface, and a third surface facing opposite the first surface. The extent to which the fill material engages the second surfaces of the microelectronic substrate can be controlled so that the fill material engages a portion of the second surfaces extending from the first surface to a point about 60% to about 70% of the distance from the first surface to the third surface of the microelectronic substrate.
The invention is also directed toward a microelectronic substrate assembly. In one embodiment, the assembly includes a microelectronic substrate having a substrate surface and at least one bond member extending away from the substrate surface and configured to bond to a support member. A volume of uncured fill material is attached to the substrate surface and to the bond member, with the fill material having an exposed surface to engage the support member. In another aspect of the invention, the microelectronic substrate and the bond member are attached to the support member and the fill material has a thixotropic index of from about four to about six when uncured.